This invention relates to LED lamps and to arrangements for mounting LED lamps on a substrate, and particularly to an arrangement for mounting LED lamps on a direction hoard, a signboard, or another LED display board.
The principal part of an LED dot matrix display unit which is used as a direction board, a signboard, or any other LED display board, which may he installed either indoors or outdoors, consists of an array of LED lamps arranged in horizontal and vertical rows on a substrate.
Each of the LED lamps in such an LED display unit has a structure conforming to its intended use. One such structure is shown in FIG. 5 by way of example. In the particular arrangement shown in FIG. 5, an LED lamp 50 has three leads 51-53 in the form of lead pins arranged to have their lower end portions soldered to a substrate. A concave reflecting plate 51a is formed at the upper end of the first lead 51 which is disposed at the center of the three lead pins.
Two LED chips 54 and 55 are bonded onto the reflecting plate 51a, and the LED chips 54 and 55 are also joined to the other leads 52 and 53 on opposite sides by two wires 52w and 53w, respectively. In the illustrated arrangement, the reflecting plate 51a is elongated in one direction to provide a long and narrow shape, and the two LED chips 54 and 55 are spaced in the direction of elongation of the reflecting plate 51a. The reflecting plate 51a is encapsulated in a sealing body 56 such as a body of transparent resin, colored transparent resin, dispersing agent containing resin, or the like.
Generally, the two LED chips 54 and 55 are chips which luminesce in different colors. For example, to provide a display in which the color of the LED lamp 50 resulting from the luminescence of both LED chips 54 and 55 is orange, the LED chip 54 has a red luminescent color, and the other LED chip 55 has a green luminescent color.
A plurality of such LED lamps 50 are arranged in horizontal and vertical rows on a substrate so as to constitute a unit module of the above-mentioned LED dot matrix display unit. A plurality of such unit modules are then arranged in horizontal and vertical rows adjacent to each other so as to provide a display unit. In use, this display unit is mounted on the wall of a building or the like and the operation of the arrays of LED lamps 50 is controlled electrically so that letters, drawings and other information are displayed on the exposed surface of the display unit.
When the display color of the LED lamp 50 appears orange by using a red LED chip 54 and a green LED chip 55 as in the above example, however, it is necessary to avoid a change in the display color of the LED lamp 50 when it is viewed from different directions.
In order to avoid such a change in color when the display unit is mounted vertically, each LED lamp 50 must be mounted on a substrate 57 so that the two LED chips 54 and 55 are spaced in the vertical direction as shown in FIG. 6.
However, when an array of such LED lamps 50 is disposed in such a manner, there is a problem because the upper lead 52 of one LED lamp 50 and the lower lead 53 of the adjacent LED lamp 50 are so close, as shown by the letter A in FIG. 6, that they can interfere with each other and cause a short circuit. This problem presents a serious obstacle to reduction of the spacing between adjacent LED lamps 50 on the substrate 56 in order to improve the visual appearance of the display unit.
To avoid this problem, a conventional arrangement has LED lamps 50 which are oriented in a display unit so that the direction of the spacing between the pairs of LED chips 54 and 55 is inclined from the vertical as shown in FIG. 7. In this way, a short circuit is prevented from occurring between adjacent pairs of LED lamps 50.
If this arrangement is used, however, there is a basic problem in the display because the display color of the LED lamp 50 changes in accordance with the direction of viewing as mentioned above. In particular, a display unit having the arrangement shown in FIG. 7 has a display color which is strong in green if the display unit is viewed from the right, while the display unit has a display color which is strong in red if the display unit is viewed from the left. Therefore, there is not only the problem that the uniformity of color is spoiled because of the variation in color depending on the direction of viewing, but also the problem that the display looks unattractive or the appearance is degraded.